Synthesis and nonlinear optical absorption properties of two new conjugated ferrocene-bridge-pyridinium compounds

Article abstract of DOI:10.1016/j.tet.2007.06.058

Two electron donor-π-acceptor (D-π-A) chromospheres, with ferrocene as the electron donor and pyridinium as the electron acceptor, were synthesized. The nonlinear optical absorption (NOA) properties in the solution state were investigated by the Z-scan technique. Both compounds exhibited reverse saturable absorption (RSA) and optical limiting effect under nanosecond pulse irradiation.

Full text of DOI:10.1016/j.tet.2007.06.058

Tetrahedron 63 (2007) 9188–9194
Synthesis and nonlinear optical absorption properties of two new
conjugated ferrocene-bridge-pyridinium compounds
Fan Yang,a Xiu-Ling Xu,a Yong-Hua Gong,a Wen-Wei Qiu,a Zhen-Rong Sun,b Jin-Wei Zhou,a
Pierre Audebertc and Jie Tanga,
*
aInstitute of Medicinal Chemistry, Department of Chemistry, East China Normal University, Shanghai 200062, China
bKey Laboratory of Optical and Magnetic Resonance Spectroscopy, East China Normal University, Shanghai 200062, China
c
´
P.P.S.M., UMR 8531, Ecole Normale Superieure de Cachan, Cachan 94235, France
Received 7 March 2007; revised 18 June 2007; accepted 19 June 2007
Available online 23 June 2007
Abstract—Two electron donor–p-acceptor (D–p-A) chromospheres, with ferrocene as the electron donor and pyridinium as the electron
acceptor, were synthesized. The nonlinear optical absorption (NOA) properties in the solution state were investigated by the Z-scan technique.
Both compounds exhibited reverse saturable absorption (RSA) and optical limiting effect under nanosecond pulse irradiation.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
second harmonic generation (SHG) efficiencies of
crystallized organic salts of (E)-1-ferrocenyl-2-(1-methyl-
4-pyridinium)ethylene showed strong dependence on the
counterions.12 By modifying the ferrocenyl group and
changing the N-alkyl chain length, Davies investigated the
variation of NLO properties of 4-substituted pyridinium.13
Although the NLO properties of 4-substituted pyridinium
system, with ferrocenylethenyl group as the electron
donor, have been studied thoroughly, the investigation of
NLO properties, especially NOA properties of the poly-
substituted pyridinium has rarely been reported. In this
paper, two new poly-substituted conjugated ferrocene-
bridge-pyridinium compounds were synthesized (Scheme
1), and the NOA property and optical limiting effect were
studied by Z-scan technique.
Nonlinear optical absorption (NOA) has shown its potential
applications in optical information storage, laser radiation
protection, and laser mode-locked. Thus searching for
NOA materials has gradually become an interesting field.
Organometallic compounds are promising candidates for
nonlinear optical materials.1,2 Since the second-order non-
linear optical (NLO) properties of ferrocene derivatives
was first reported by Green et al. in 1987,3 the studies
of organometallic compounds with ferrocenyl group have
aroused more attention,4–11 in which the more interesting
compound was the substituted pyridinium with ferrocenyl-
ethenyl group. As demonstrated by Marder et al., the powder
Fe
+
+
N
S
S
N
I-
Fe
Fe
CH3
Fe
Fe
CH3
I-
1
2
Scheme 1.
Keywords: Ferrocene; Pyridinium; Electron donor–acceptor; Reverse saturable absorption; Optical limiting effect.
* Corresponding author. Fax: +86 21 62232100; e-mail: jtang@chem.ecnu.edu.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.06.058

F. Yang et al. / Tetrahedron 63 (2007) 9188–9194
9189
2. Results and discussion
2.1.2. N-Methyl-2,6-di[(5-(E)-ferrocenylethenyl)thio-
phene-2-yl-(E)-ethenyl]pyridinium iodide (2). The syn-
thesis of compound 2 may be realized, similar to compound
1, by condensation of trans-5-(E)-(2-ferrocenylethenyl)thio-
phene-2-carbaldehyde (10) with N-methyl-2,6-dimethyl
pyridinium iodide (15). With regard to the synthesis of com-
pound 10, Thomas reported a method, which is shown in
Scheme 3.16,17
2.1. Synthesis
2.1.1. N-Methyl-2,4,6-tri[(E)-ferrocenylethenyl]pyridi-
nium iodide (1). According to the reported method of con-
densation of N,2-dimethyl pyridinium iodide with
aldehyde,14 we tried to synthesize compound (1) by the con-
densation of excess of ferrocenecarboxaldehyde with N-
methyl-2,4,6-trimethyl pyridinium iodide in methanol. It
was found that the produced product (1) concreted in the re-
action system for the insolubility in methanol, which caused
the stirring difficulty, so the reaction could not be carried out
completely. However, altering the solvent with CH3OH/
CHCl3 (3:1, v:v), the reaction could proceed successfully.
Recrystallized twice with the component solvent, N-methyl-
2,4,6-tri[(E)-ferrocenylethenyl]pyridinium iodide (1) was
obtained with high purity.
However, when preparing compound 10 from (E)-2-ferroce-
nylethenylthiophene (9) by Vilsmeier reaction, the products
were complicated although compound 9 was converted com-
pletely, and only little desired product was obtained. Maybe
the cyclopentyldiene rings of the ferrocene group could also
be hydroformylated, which resulted in the complication
of product. Hence, we designed a new route as follows
(Scheme 4).
Thiophene-2-carbaldehyde (5), prepared by formylation of
thiophene, was brominated to give 5-bromothiophene-2-carb-
aldehyde (11), then compound 11 was reduced, and the
product (5-bromothiophen-2-yl)methanol (12) was trans-
formed to the corresponding Wittig reagent. Finally, the
Wittig reagent reacted with ferrocenecarboxaldehyde to
give a mixture of cis- and trans-5-bromo-2-(2-ferrocenyl-
ethenyl)thiophene (14). The cis-isomer could be conveniently
transformed to the trans-isomer by adding a small amount of
iodine, and the trans-5-bromo-2-(2-ferrocenylethenyl)thio-
phene could be easily formylated. After purification, trans-
5-(E)-(2-ferrocenylethenyl)thiophene-2-carbaldehyde (10)
was obtained with 60% yield and high purity. Catalyzed
by base, condensation of compound 10 with N-methyl-2,6-
dimethyl pyridinium iodide (15) was carried out to give
the final product N-methyl-2,6-di[(5-(E)-ferrocenylethe-
nyl)thiophene-2-yl-(E)-ethenyl]pyridinium iodide (2) in
79% yield.
N-Methyl-2,4,6-trimethyl pyridinium iodide was prepared
by reaction of 2,4,6-trimethylpyridine with methyl iodide,
and ferrocenecarboxaldehyde was prepared according to
the literature (Scheme 2).15
CHO
AlCl3
+
CH(OEt)3
Fe
Fe
3
3
+
CH3I
+
1
N
N
I-
CH3
4
Scheme 2.
CH2OH
.
CH2P+Ph3Br-
S
S
S
S
CHO
POCl3
NaBH4
PPh3 HBr
DMF
5
6
7
S
S
POCl3
DMF
CHO
S
t-BuOK
3
CH=PPh3
Fe
10
Fe
8
9
Scheme 3.
S
S
S
Br
.
S
+
CHO
Br
Br
CH2OH
CHO
NaBH4
PPh3 HBr
PPh3Br-
Br2
5
12
13
11
S
S
t-BuOK
3
Br
CHO
nBuLi
DMF
Fe
Fe
10
14
10
+
CH3I
+
2
N
N
I-
CH3
15
Scheme 4.

9190
F. Yang et al. / Tetrahedron 63 (2007) 9188–9194
2.2. Nonlinear optical absorption properties
For this purpose, the UV–vis absorption spectra of com-
pounds 1 and 2 were measured (Fig. 2). Absorption para-
meters obtained are listed in Table 1.
Nonlinear optical absorption (NOA) measurements of these
two compounds were carried out at nanosecond scale by
open aperture Z-scan technique. Figure 1 exhibits the nor-
malized Z-scan transmittance of 2ꢀ10ꢁ4 mol/L solutions
of compounds 1 and 2 in DMF. The nanosecond laser source
was a Q-switch locked Nd:YAG (1064 nm) with a second
harmonic generation of 532 nm, pulse width of 12 ns, repeat
frequency of 10 Hz, average pulse energy of 0.114 mJ and
peak irradiance of 0.670 GW/cm2.
Table 1. UV–vis absorption parameters of compounds 1 and 2
Samples
1
2
lmax
A (lmax
A (l¼532 nm)
370
2.835
1.087
225
1.992
1.414
)
As the sample was moved away from the focus point, the
transmittances of both compounds were nearly a flat line,
which displayed the linear absorption under weak light irra-
diation. As the samples were moved close to the focus point,
the transmittances decreased as the laser irradiance in-
creased. At the focus point (Z¼0) where the laser irradiance
reached maximum, the normalized transmittance decreased
to minus. These results indicated that both compounds 1 and
2 were of obvious reverse saturation absorption (RSA).
It was obvious that the lmax of both compounds appeared in
the UV region, and the lmax of compounds 1 and 2 were at
370 nm and 225 nm, respectively. Except for the strong
peak in the UV region, both of them had a peak in the
520–540 nm regions with moderate strength, which might
be resulted from the characteristic internal charge transfer
(ICT) transition from the donor of ferrocene unit to the
poly-substituted pyridinium unit. Thus, with 532 nm laser
irradiation, both compounds 1 and 2 exhibited significant
reverse saturation absorption properties.
For conjugated D–p-A systems, the distortion of molecular
cloud under strong laser irradiation was the reason for non-
linear phenomena. For conjugated p systems of organic
compounds, the conjugation behavior of the molecular
bone can be easily characterized by UV–vis spectroscopy.
2.3. Optical limiting effect
The nonlinear optical limiting effect, or simply the optical
limiting effect, refers to the fact that the transmittance of
(a)
(b)
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
1.05
D
E
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
-10 -8
-6
-4
-2
0
2
4
6
8
-15
-10
-5
0
5
10
Z (mm)
Z (mm)
Figure 1. The reverse saturation absorption of compounds 1(a) and 2(b).
(c)
(d)
3.0
1.0
D
C
2.5
2.0
1.5
1.0
0.5
0.0
0.8
0.6
0.4
0.2
0.0
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
800
wavelength (nm)
wavelength (nm)
Figure 2. UV–vis absorption spectra of compounds 1(c) and 2(d).

F. Yang et al. / Tetrahedron 63 (2007) 9188–9194
9191
a material decreases with increase of the input light fluence
or intensity, which has attracted considerable attention be-
cause of its potential application in the fields of laser protec-
tion and optical communication. Using the Nd:YAG laser
with pulse width of 12 ns as the light source, the optical lim-
iting properties of both compounds were measured under
three different concentrations. The input–output energy
plots are shown in Figure 3.
dependence on the irradiance of the laser. As light irradiance
increased continuously, transmittance started to decrease. In
other words, both compounds showed optical limiting effect.
For compound 2, the optical limiting threshold value (EL),
defined as the light irradiance at which transmittance turns
down in the T–I plot, was about 3.7 mJ. For compound 1,
the EL value was less than 1 mJ. Except for the difference
in the EL values, the strengths of optical limiting effects
for the two compounds were different too. At light irradiance
of 7 mJ, the transmittance decrease for compound 1 was
about 40% as compared with that of the linear region. For
compound 2, the decrease was only 14.6%. The optical lim-
iting effect for compound 1 was far more significant than that
of compound 2. This behavior was consistent with the
strength of their reverse saturation absorption behavior.
For the two compounds at same concentration, the transmit-
tance for compound 1 was larger than that of compound 2 in
the linear region. When light irradiance increased, the trend
of the change of T–I plot for a given compound was about the
same. Based on the results obtained above, we concluded
that compound 1 was a better optical limiting material for
the two compounds synthesized.
When the input laser energy was weak, the output energy in-
creased linearly as the input energy increased. The absorp-
tion coefficient was independent on the laser irradiance
(I0). As the input laser irradiance (I0) increased continuously,
the increase in output energy deviated from the linear rela-
tionship, which was less than that predicted from the slop
of the linear region. This meant that the absorption coeffi-
cients of the samples increased as the input laser irradiance
(I0) increased.
Transmittance of the sample was obtained by dividing the
output energy by input energy. The variation of transmit-
tance (T) against light irradiance (I) provides a measurement
of optical limiting property of the sample. Figure 4 exhibits
the plots of transmittance of compounds 1 and 2 against light
irradiance at different concentrations.
Comparing the molecular structures of compound 1 with 2,
both of them are of the same electron acceptor, the difference
is that compound 1 has got three ferrocenyl groups as the elec-
tron donor. The increase of the electron-donating group is
favored to the delocalization of p-electron, and the electron
At low light irradiance, the T–I plot was flat, that indicated
a linear optical behavior. The transmittance had no
(e)
(f)
3.4
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
3.2
1/2
3.0
1/2
1/4
1/8
1/4
2.8
1/8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
E
D
-1
0
1
2
3
4
5
6
7
8
-1
0
1
2
3
4
5
6
7
8
Incident Energy (mJ)
Incident Energy (mJ)
Figure 3. Optical limiting effect of compounds 1(e) and 2(f).
(g)
(h)
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.75
1/2
1/4
1/8
1/2
1/4
1/8
D
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
E
-1
0
1
2
3
4
5
6
7
8
-1
0
1
2
3
4
5
6
7
8
Incident Energy (mJ)
Incident Energy (mJ)
Figure 4. Transmittance–input energy curves for compounds 1(g) and 2(h).

9192
F. Yang et al. / Tetrahedron 63 (2007) 9188–9194
cloud is looser, which is susceptible to distortion when excited
by laser light. So the nonlinear optical absorption effect of
compound 1 is moreremarkable. Asforcompound2, although
a thiophenethenyl group was inserted in the bridge as the chro-
mophore, both of the reverse saturation absorption and optical
limiting effects areworse than that of compound 1. Clearly, in-
crease in the number of the electron-donating group may be
more effective for promotion of the nonlinear optical absorp-
tion properties of the pyridinium salts.
(1.5 mL) was added. The mixture was refluxed for 6 h under
the protection of N2, and then cooled slowly. The solid was
collected, and crystallized twice with methanol/chloroform,
3.68 g of violet plate crystal product was obtained, yield
86.4%. Anal. Calcd for C42H38Fe3IN: C, 59.22; H, 4.37;
N, 1.65. Found: C, 59.45; H, 4.76; N, 1.68; UV:
1
lmax¼370 nm (CH3CN); H NMR (DMSO-d6, d): 4.16 (s,
3H, NCH3), 4.18–4.21 (m, 15H, C5H5+2C5H5), 4.51–4.53
(m, 6H, C5H2+2C5H2), 4.78–4.81 (m, 6H, C5H2+2C5H2),
6.83 (d, 1H, J¼15.9 Hz, CH]CH), 6.99 (d, 2H,
J¼15.0 Hz, 2CH]CH), 7.58 (d, 2H, J¼15.0 Hz,
2CH]CH), 7.75 (s, 2H, C5H2N), 7.80 (d, 1H, J¼15.9 Hz,
CH]CH).
3. Conclusion
1. We designed two new poly-substituted conjugated
ferrocene-bridge-pyridinium compounds, N-methyl-
2,4,6-tri[(E)-ferrocenylethenyl]pyridinium iodide (1)
and N-methyl-2,6-di[(5-(E)-ferrocenylethenyl)thiophene-
2-yl-(E)-ethenyl]pyridinium iodide (2). Compound 1
could be synthesized conveniently by the reaction of fer-
rocenecarboxaldehyde with N-methyl-2,4,6-trimethyl
pyridinium iodide. To synthesize compound 2, a new
route for a key intermediate, trans-5-(E)-(2-ferrocenyl-
ethenyl)thiophene-2-carbaldehyde (10), was designed
and successfully actualized. Then compound 10 reacted
with N-methyl-2,6-dimethyl pyridinium iodide to give
the target compound.
2. The nonlinear optical absorption of the two compounds
was investigated by the Z-scan technique. Both com-
pounds exhibited reverse saturable absorption (RSA)
and optical limiting effect under nanosecond pulse irra-
diation, and compound 1 showed more obvious optical
limiting effect.
4.3. Preparation of N-methyl-2,6-di[(5-(E)-ferrocenyl-
ethenyl)thiophene-2-yl-(E)-ethenyl]pyridinium iodide (2)
4.3.1. Thiophene-2-carbaldehyde (5). To a three-necked
round bottom flask were added 16 mL (0.19 mol) of thio-
phene, 16.3 mL (0.21 mol) of anhydrous DMF and 40 mL
of anhydrous chloroform. The solution was cooled to
w0 ꢂC, and 19.6 mL of POCl3 (0.21 mol) was added drop-
wise with stirring. The solution was then warmed to room
temperature followed by refluxing for 4 h, then cooled to
room temperature and poured into ice water. The water layer
was extracted with dichloromethane (3ꢀ50 mL), and the or-
ganic layers were combined, washed with water (2ꢀ50 mL)
and saturated aqueous sodium bicarbonate (2ꢀ50 mL), and
then dried over MgSO4. After evaporation of the solvent,
25 g pure thiophene-2-carbaldehyde (5) was obtained by
fractional distillation as a colorless liquid (70% yield; bp
108–112 ꢂC/53 mmHg, lit.17 44–45 ꢂC/1 mmHg).
4.3.2. 5-Bromothiophene-2-carbaldehyde (11).18 To an
ice-cooled solution of thiophene-2-carbaldehyde (16.2 g,
0.14 mol) and NaHCO3 (12.6 g, 0.15 mol) in dry chloro-
form, bromine (7.4 mL, 0.14 mol) was added dropwise
with stirring, and the mixture was left at room temperature
for 2 h, then poured into water. After separation, the water
layer was extracted with dichloromethane (3ꢀ20 mL), and
the organic layers were combined, washed with saturated
aqueous sodium bicarbonate and water, and then dried
with MgSO4. After a short column (silica, 1:20 ethyl ace-
tate/petroleum ether) 21.4 g of 11 was obtained as a colorless
liquid (80% yield). 1H NMR (CDCl3, d): 9.78 (s, 1H), 7.52
(d, 1H, J¼4.0 Hz), 7.19 (d, 1H, J¼4.0 Hz).
4. Experimental
4.1. General
NMR, UVand elemental analysis were carried out on Bruker
AM-500 MHz, UnicamHelios a UV spectrophotometer and
Elementar ZAROIO-EL elemental analyzer. Except spe-
cially stated, all reagents used were analysis grade and
were treated before use. Ferrocenecarboxaldehyde was pre-
pared according to the literature.15
4.2. Preparation of N-methyl-2,4,6-tri[(E)-ferrocenyl-
ethenyl]pyridinium iodide (1)
4.3.3. (5-Bromothiophen-2-yl)methanol (12). To an ice-
cooled solution of 15 g (0.079 mol) of 5-bromothiophene-
2-carbaldehyde (11) in 50 mL of methanol, NaBF4 (6 g,
0.16 mol, 2 equiv) was added portionwise, and the mixture
was stirred at room temperature for about 4 h. After evapo-
ration of the solvent, dichloromethane was added to dissolve
the residue and the insoluble salts were filtered. After a short
column chromatography (silica, 1:10 ethyl acetate/petro-
leum ether) 11 g of 12 was obtained as a colorless liquid
4.2.1. N-Methyl-2,4,6-trimethylpyridinium iodide.
Methyl iodide (7.10 mL, 0.114 mol) was added dropwise to
a solution of 2,4,6-trimethylpyridine (4.85 g, 0.04 mol) in
dichloromethane at 0 ꢂC. After stirring at room temperature
overnight, the solvent was evaporated to give a white solid,
which was recrystallized in methanol to give 8.95 g product
1
as white crystal with yield of 84.7%. Mp 213–215 ꢂC; H
NMR (D2O, d): 2.45 (s, 3H), 2.68 (s, 6H), 3.95 (s, 3H),
7.51 (s, 2H).
1
(73% yield). H NMR (CDCl3, d): 6.92 (d, 1H, J¼3.7 Hz),
6.76 (d, 1H, J¼3.7 Hz), 4.75 (s, 2H), 1.73 (s, br, 1H).
4.2.2. N-Methyl-2,4,6-tri[(E)-ferrocenylethenyl]pyridi-
nium iodide (1). N-Methyl-2,4,6-trimethylpyridinium
iodide (1.32 g, 5 mmol), ferrocenecarboxaldehyde (6.42 g,
30 mmol), methanol (20 mL), and chloroform (10 mL)
were added into a round bottom flask, then piperidine
4.3.4. [(5-Bromothiophen-2-yl)methyl]triphenylphos-
phonium bromide (13).19 (5-Bromothiophen-2-yl)meth-
anol (5 g, 0.026 mol), triphenylphosphine hydrobromide
(8.9 g, 0.026 mol) and 40 mL of anhydrous chloroform

F. Yang et al. / Tetrahedron 63 (2007) 9188–9194
9193
were added into a flask equipped with a condenser. After re-
fluxing for 1 h, the condenser was replaced by distillation
equipment. The solvent was distillated off slowly until dry-
ness for 1 h. The crude product was then dissolved in chlo-
roform, and the pure phosphonium bromide 13 was
recovered by precipitation with the addition of diethyl ether,
washed twice with ether, and isolated by filtration (12.8 g,
96%).
4.3.8. N-Methyl-2,6-di[(5-(E)-ferrocenylethenyl)thio-
phene-2-yl-(E)-ethenyl]pyridinium iodide (2). To
a
50 mL flask equipped with a condenser, 31 mg (0.12 mmol)
of N-methyl-2,6-dimethyl pyridinium iodide (15), 150 mg
(0.37 mmol, 3 equiv) of trans-5-(E)-(2-ferrocenylethe-
nyl)thiophene-2-carbaldehyde (10), 25 mL of anhydrous
methanol and two drops of piperidine were added. After
refluxing for 4 h under N2 atmosphere, the solution was
cooled to room temperature. The solvent was evaporated
and the residue was passed through a column chromato-
graphy (silica, 1:15 methanol/dichloromethane) to give
84 mg of 2 as a dark violet solid (79%). Anal. Calcd for
C42H36Fe2INS2: C, 58.83; H, 4.23; N, 1.63. Found: C,
4.3.5. trans-5-Bromo-2-(2-ferrocenylethenyl)thiophene
(14).20 [(5-Bromothiophen-2-yl)methyl]triphenylphospho-
nium bromide (5.18 g, 0.01 mol) was dissolved in 30 mL
of dry THF under N2 atmosphere and cooled tow0 ꢂC. Po-
tassium tert-butoxide (1.39 g, 0.012 mol) was added slowly
while stirring the solution vigorously. The generated deep
red solution was stirred at 0 ꢂC for another 30 min, and
then brought to room temperature. Ferrocenecarboxalde-
hyde (2.14 g, 0.01 mol) was added portionwise and the mix-
ture was heated to reflux for 4 h. The solution was poured
into ice water and extracted with diethyl ether. The ether
layer was washed with brine and dried with MgSO4. A mix-
ture of cis- and trans-5-bromo-2-(2-ferrocenylethenyl)thio-
phene was obtained by a short column chromatography
(silica, 1:6 dichloromethane/petroleum ether) as a red solid
(3.2 g, 86%).
1
58.54; H, 4.52; N, 1.56; UV: lmax¼225 nm (CH3CN); H
NMR (DMSO-d6, d): 8.34 (t, 1H, J¼8.1 Hz), 8.20 (d, 2H,
J¼8 Hz), 7.97 (d, 2H, J¼15.5 Hz), 7.51 (d, 2H, J¼3.7 Hz),
7.21 (d, 2H, J¼15.5 Hz), 7.16 (d, 2H, J¼3.7 Hz), 7.03 (d,
2H, J¼15.8 Hz), 6.88 (d, 2H, J¼15.8 Hz), 4.63 (s, 4H),
4.40 (s, 4H), 4.21 (s, 3H), 4.17 (s, 10H); 13C NMR
(DMSO-d6, d): 152.67, 147.32, 137.42, 135.04, 133.47,
130.32, 126.27, 122.68, 118.62, 116.29, 81.87, 69.63,
69.12, 67.16, 41.25; HRMS m/z: 730 [C42H36Fe2NS2]+.
Acknowledgements
Recrystallization of the mixture from hot hexane gave 1.3 g
of pure trans-5-bromo-2-(2-ferrocenylethenyl)thiophene
(14). The mixture recovered from filtrate (cis/trans¼4:1)
was treated with 0.01 equiv of I2 in refluxing toluene under
N2 atmosphere for 1 h to give quantitative pure trans-prod-
This project was supported by National Natural Science
Foundation of China (no. 20572026) and Natural Science
Foundation of Shanghai (no. 05ZR14042).
1
References and notes
uct. H NMR (CDCl3, d): 6.90 (d, 1H, J¼4.0 Hz), 6.64 (q,
2H, J¼16.0 and 3.0 Hz), 6.56 (d, 1H, J¼16.0 Hz), 4.47 (s,
2H), 4.34 (s, 2H), 4.18 (s, 5H).
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10 was obtained as a violet solid (60%). H NMR (CDCl3,
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J¼4.0 Hz), 6.96 (d, 1H, J¼16.0 Hz), 6.79 (d, 1H,
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